This invention relates to a f-theta scan lens for an optical scanning system and, more particularly, a f-theta scan lens for a dual wavelength raster output scanning system (ROS).
Raster output scanners conventionally have a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep an intensity modulated beam across a photosensitive medium in a line scanning direction (also know as the scan direction) while the recording medium is being advanced in an orthogonal or "process" direction (also known as the slow scan direction) such that the beam scans the recording medium in accordance with a raster scanning pattern.
The raster output scanners in general have F-theta lenses in the optical path between the rotating polygon mirror and the scanning surface of the photosensitive medium. The light beam will be deflected at a constant angular velocity from the rotating mirror which the F-theta lens optically modifies to scan the surface at a constant velocity. These ROS systems in the past have employed F-theta scan lenses designed for use with monochromatic light sources and, therefore, were not designed to compensate for the effects of chromatic aberration.
A multiple beam, multiple wavelength ROS 10 of the prior art is shown in FIG. 1. The multiple beam laser 12 consisting of a pair of laser diodes (not shown) emits two beam 14 and 16, each having a different wavelength. The beams 14 and 16 are collimated by collimator 18. The light beams then continue through a single element cross-scan cylinder lens 20 which focuses them in the cross-scan direction onto the reflective facet 22 of the rotating polygon mirror 24. After reflection and scanning from the facet 22, the two beams 14 and 16 pass through the F-theta scan lens 26. The F-theta scan lens is shown as a three-element lens 28, 30 and 32.
Subsequently, the wobble correction optical elements 34 reimage the focused beams 14 and 16 reflected from polygon facet 22 onto the photoreceptor plane 36 at predetermined positions, independently of the polygon angle error or tilt of the facet 22. The wobble correction optics 34 can consist of a pair of fold mirrors 38 and 40 and a cylindrical mirror 42 to image the focused beams 14 and 16 on the plane 6. Such compensation is possible because the focused beams are stationary "objects" for the F-theta scan lens 26 and the wobble correction optics 34. Although, due to polygon tilt, or wobble, the beams 14 and 16 are reflected to different positions of the post-polygon optics aperture for each different facet of the rotating polygon, the beams 14 and 16 are imaged to the same positions on the PR plane 36.
"Bow" is a measure of distortion in the cross-scan direction of the scan line from one end of the scan to the other. Bow may be calculated by taking the average of the cross-scan heights at the extreme ends of the scanline then subtracting the cross-scan height at the center of scan. In a multiple beam system, each light source emitting a beam has its own bow curve. It is the maximum difference in the bow curves between the multiple light sources in a given system that defines the "differential bow".
The optical design must achieve F-theta correction in the optics to ensure the "scan linearity." Scan linearity is the measure of how equally spaced the spots are written in the scan direction across the entire scanline. Typical scan linearity curves start at zero position error at one end of a scan having a positive lobe of position error, cross the center of scan with zero position error and then have a negative lobe of position error toward the other end of the scan. Scan linearity curves may have image placement errors of zero at several locations across the scanline. Ideally, the curve would be at zero across the entire scanline.
There are two color aberrations in the lens of a multiple beam, multiple wavelength ROS that must be corrected or compensated for: (1) longitudinal or axial chromatic aberration and (2) tangential or lateral chromatic aberration. Axial chromatic aberration causes light of different wavelengths to come to a focus at different distances back from the lens. Lateral chromatic aberration is the variation in image height of focused spots of light having different wavelengths, taken at a specified focal plane.
With the advent of multiple wavelength laser diodes as light sources, multiple beam ROS optical systems that use these light sources must have scan lenses that enable precise and accurate scanning with beams of different wavelengths. In these lenses, both axial chromatic aberration and lateral chromatic aberration must be corrected to a high degree. Lateral chromatic aberration, also known as chromatic aberration of magnification, must be corrected to within a small fraction of a pixel throughout the entire scan to be effective.
Accordingly, there is a need for a diffraction limited F-theta scan lens for a multiple beam, multiple wavelength ROS that exhibits very little lateral chromatic aberration and very little field curvature along the main scanning direction and that has a longitudinal chromatic aberration that is smaller than the depth of focus of the beam at the image plane.